LEDs are solid-state devices that produce light when electrical current flows therethrough. They are typically polarized, low-voltage devices, although they may be combined into arrays requiring higher voltages. Despite having minimum and operating voltage specifications, LEDs are typically specified and regulated with respect to the amount of current required for operation. Unlike conventional loads that require a fixed voltage, LEDs typically require a near-constant current for ideal operation.
An LED light source that runs directly from a commercial alternating current (AC) 120 volt (V) power supply (“line voltage”) usually requires a current-regulated ballast circuit for ideal operation. This circuit is responsible for both current and temperature regulation, and should be immune to voltage spikes and other noise on the AC line within a predetermined design range.
A non-isolated linear LED driver can provide excellent affordability and reliability and can be made to have a very small profile. Without the additional bulk and cost of a transformer, inductors, and, particularly, electrolytic capacitors, the driver can be made very compact and the detrimental effects of capacitor aging can be minimized.
However, without electrolytic capacitors to filter (or “buffer”) the line voltage, the circuit is subject to any noise that might be present on the incoming power lines (or “mains”). Particularly, noise in the visible frequency range, which extends up to about 100 Hz, may be detrimental to the performance of LEDs.
There are several ways that ideal line conditions may be disturbed such that alternating cycles and rising or falling edges may become asymmetrical from their counterparts. These conditions may conspire to create an undesirable visible flicker in the LED light output.
For example, if an LED load draws current from an AC power source that is also connected to a circuit having a poor power factor, the rising and falling edges of the line voltage may become mismatched or asymmetrical. Other sources of line distortion, such as fluorescent lighting ballasts, can easily cause mismatches between edges within the same cycle.
Wall dimmers often use inexpensive DIAC-TRIAC circuits. This type of wall dimmer implements phase-cut style dimming where the rising edge is delayed for some time according to the setting of the adjustment potentiometer. These devices often exhibit asymmetrical behavior in the different operating quadrants of the TRIAC which may result in each alternative half-cycle on the line having slightly differing durations. When a wall dimmer using phase-cut style dimming is connected to the same AC power source as an LED driver, this difference in half-cycles may manifest itself in the LED light output as a disturbing flicker.
One solution for dealing with line voltage variations is to integrate the voltage across a sense resistor provided in series with the LED load and apply the output of the integration (in the form of a voltage) to the gate of a metal-oxide-semiconductor field-effect transistor (“MOSFET”) provided in series with the LED load. This works to an extent, but the time constant necessary for good regulation is longer than several cycles of the line voltage; therefore, this type of implementation cannot compensate for short-term fluctuations such as half-cycle or quarter-cycle asymmetry.
In order to compensate for such short-term fluctuations, a massive electrolytic capacitor with a rating about equal to the voltage drop across the LED load may be connected in parallel with the LED array. However, due to their large size and cost, utilizing an electrolytic capacitor in such a way can be detrimental to the compactness and/or price, and thus marketability, of a LED driver.